This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-97653 filed on Mar. 31, 2000.
1. Field of the Invention
The present invention relates to a motor having a worm and a worm wheel.
2. Description of Related Art
As shown in FIG. 5, one previously proposed motor includes a motor main body 81 and a speed reducing unit 82 secured to the motor main body 81. The motor main body 81 includes a generally cylindrical yoke housing 83. The motor main body 81 further includes a rotor 84 substantially contained in the yoke housing 83. The rotor 84 includes a rotatable shaft 85, an armature 86 secured to the rotatable shaft 85, and a commutator 87. A base end portion of the rotatable shaft 85 is rotatably supported by a radial bearing 88 arranged at a base of the yoke housing 83. In a base end surface of the rotatable shaft 85, there is formed a ball receiving recess 85a in which a metal ball 89 is partially received. Part of the ball 89 that protrudes from the ball receiving recess 85a engages a bottom wall of the yoke housing 83 to act as a thrust bearing that supports a load exerted in an axial direction of the rotatable shaft 85. A worm 90 is provided at a distal end portion of the rotatable shaft 85.
The speed reducing unit 82 includes a gear housing 91 secured to an open end of the yoke housing 83, a worm wheel 92 contained and rotatably supported in the gear housing 91, and the like. In the gear housing 91, there is formed a shaft receiving recess 91a that receives a portion of the rotatable shaft 85 (worm 90) extending out from the yoke housing 83. A radial bearing 93 is arranged at an open end of the shaft receiving recess 91a. A middle portion of the rotatable shaft 85 is rotatably supported by the radial bearing 93. The worm wheel 92 is meshed with the worm 90 of the rotatable shaft 85.
When the rotor 84 is rotated, a rotational speed of the rotor 84 is reduced through the speed reducing unit 82 (worm 90 and worm wheel 92). Then, the rotational force of the speed reducinq unit 82 is outputted from the motor through an output shaft 94 connected to the worm wheel 92.
The above motor is designed to rotate the worm wheel 92 shown in FIG. 5 only in a counterclockwise direction and is used, for example, as a motor of wiper system.
By way of example, during operation of the wiper system, if there is some resistance, such as a resistance exerted by snow located on a wiping surface of a vehicle front glass, a large load larger than a load normally applied on the output shaft 94 is quickly applied on the output shaft 94 via a link mechanism of the wiper system which links the output shaft 94 to wipers of the wiper system. Thus, a large reaction force is applied on the rotatable shaft 85, thereby causing movement of the rotatable shaft 85 toward its base end side (right side in FIG. 5). However, this movement of the rotatable shaft 85 is limited by the ball 89, so that a large force (rotational force or inertial force of the rotor) is applied on a connection (teeth) between the worm 90 and the worm wheel 92. As a result, in the described motor, each one of the worm 90 and the worm wheel 92 needs to have a large strength that is larger than a normally required strength to avoid damage to the connection (teeth) between the worm 90 and the worm wheel 92. This results in an increase in a material cost of the worm wheel 92. This is especially true when the worm wheel 92 is made of a resin material. Also, this results in a decrease in a motor efficiency due to an increase in the weight of each rotating component. Furthermore, this results in an increase in a gear module, so that the worm wheel becomes bigger. Thus, a size of the entire gear housing needs to be increased.
Accordingly, it is an objective of the present invention to provide a motor that has a worm and a worm wheel and that can prevent application of a large force on a connection between the worm and the worm wheel.
To achieve the objective of the present invention, there is provided a motor including a motor case, a rotor and a worm wheel. The rotor is contained in the motor case and includes a rotatable shaft. The rotatable shaft has a worm that is meshed with the worm wheel. The motor further includes frictional braking force generating means for braking the rotatable shaft with friction when a thrust load applied on the rotatable shaft exceeds a predetermined value.